Composition Dependence Of Structural Parameters, Phase Stability And Magnetic Properties Of Modulated Martensites In Ni-Mn-Ga Alloys Studied By Ab Initio Calculations

As a novel type of multi-functional smart materials for potential uses in sensor and actuator devices, Ni-Mn-Ga alloys with chemical composition close to Ni2MnGa have attracted considerable attention due to their large output strain up to 10% and quick response under magnetic field.During the last two decades, numerous experimental studies have been conducted on various aspects of Ni-Mn-Ga alloys and a lot of valuable results have been achieved. According to the experimental observations, the large magnetic shape memory effect can be observed mainly in modulated martensites. However, the modulated martensite in stoichiometric Ni2MnGa and off-stoichiometric Ni-Mn-Ga alloys has been revealed to be metastable in the previous studies before 2012. Thus the modulated martensite has a tendency to further transform to non-modulated martensite.It has been established that the phase stability and the martensitic transformation temperature of Ni-Mn-Ga alloys are closely related to chemical composition. The martensitic transformation temperature increases gradually with increasing e/a value. Therefore, composition tuning through the enhancement of the e/a value is an effective route to achieve the stable modulated martensite with excellent magnetic properties in wide composition range of Ni-Mn-Ga alloy. However, due to the complicated long-period modulated structures and the difficulties in the experimental determination of the atomic moments, the in-depth corresponding research is greatly hindered.Recently, the crystal structures of the modulated martensites have been resolved by the application of the superspace theory on powder diffraction and simulation. Based on the detailed crystal information of modulated martensite, we made a first attempt to further calculate the structure stability and the magnetic properties of modulated martensites in stoichiometric Ni2MnGa and off-stoichiometric Ni-Mn-Ga alloys (Ni2+xMn1-xGa (0.05?x? 0.25), Ni2+xMnGa1-x (0.05?x?0.10), and Ni2Mn1+xGa1-x (0.05?x?0.20) alloys). All of the calculations were performed within the framework of the Density Functional Theory (DFT) using the Vienna Ab initio Simulation Package (VASP). Under the frame of superstructure, an 80-atom supercell is selected for each phase in view of a convenient comparison between the calculated results. For further comparison, the structure stability and magnetic properties of austenite and non-modulated martensite were also calculated. The innovative results are summarized as follows:1. The structural parameters, formation energy, total magnetic moment and the magnetic moment for each constituent atom in five-layered modulated martensite and seven-layered modulated martensite of stoichiometric Ni2MnGa were calculated. The phase stability and magnetic properties especially the distribution of the atomic moment along the c-axis of modulated martensites were studied. The distribution of the total magnetic moment of modulated martensites in Ni2MnGa was analyzed. The physical mechanism of the increase of the total magnetic moment when the structure changes from austenite to non-modulated martensite passing through the modulated martensite was further elucidated.2. The site occupation between the excess atoms in austenite,7M martensite and NM martensite of off-stoichiometric Ni2+xMn1-xGa (0.05?x?0.25), Ni2+x MnGa1-x (0.05?x? 0.10), and Ni2Mn1+xGa1-x (0.05?x?0.20) alloys was studied. It has been revealed that the excess atoms prefer to occupy sites that are not adjacent in the superstructure.3. The structural parameters, formation energy, total magnetic moment and the magnetic moment for each constituent atom in modulated martensite of off-stoichiometric Ni2+xMn1-xGa (0.05? x?0.25), Ni2+x MnGa1-x (0.05?x?0.10), and Ni2Mn1+xGa1-x (0.05?x?0.20) alloys were calculated. The phase stability and magnetic properties especially the distribution of the atomic moment along the c-axis of modulated martensites were studied. The evolution of the modulated martensite of Ni-Mn-Ga alloy with the change of the composition doping was discussed, and the related mechanism was analyzed.4. The results show that Ni-doping (Ni2+xMn1-xGa (0.05?x?0.25) and Ni2+xMnGa1-x (0.05?x?0.10)) relatively stabilizes the NM martensite, whereas proper Mn-doping (Ni2Mn1+xGa1-x (0.10?x?0.20)) relatively stabilizes the 7M martensite. Therefore stable 7M martensite can be achieved in wide composition range of Ni2Mn1+xGa1-x (0.10?x? 0.20) alloy, and the martensitic transformation temperature fits the practical application in the alloys with high Mn content.5. The perturbation of the magnetic moments by atom substitution is mainly located in the antisite and its neighboring elements and is dominated by their Mn environment. The magnetic moment increases if the distance between the atoms being considered and its nearest Mn neighbor decreases ("distance effect"), or the number of its nearest Mn neighbors increases ("number effect").The present work aimed at exploring the variation of the structural parameters, phase stabilities and magnetic properties of Ni-Mn-Ga alloys, which is expected to provide a solid foundation for the practical application. Moreover, the insight into physical nature of the magnetic properties would also offer theoretical guidelines for the further alloy design and performance optimization of new promising ferromagnetic shape memory alloys.